This invention relates to the detection and characterization of small particles, aerosols or other elements in a fluid stream, and more particularly to determining particle velocities, as an end in itself and for determining aerodynamic particle sizes and other particle characteristics.
Optical measurements of particle velocities are widely used in studies of gas flows and in particle dynamics. Particle size measurements are employed in a variety of fields involving the study and monitoring of pollution and contamination, respirable particle mass, and spray nozzle performance. Techniques have been developed for the empirical estimation of particle characteristics, e.g. composition, mass and index of refraction, based on velocity and size measurements.
One well-known velocity measurement technique is laser Doppler velocimetry (LDV) also known as laser Doppler anemometry. According to this approach, particles or other elements simultaneously scatter light from two coherent beams with different angles of incidence. A photodetector receives the scattered light and generates a frequency representing the heterodyne difference in Doppler shift frequencies produced by particle motion relative to the beams. LDV is useful, yet requires careful alignment of the beams. Beam coherency must be maintained. Sophisticated and expensive signal processing techniques are required to measure the short frequency bursts produced by the particles. Phase Doppler anemometry also is used in measuring the sizes of spherical droplets. This requires multiple photodetectors aligned at precise angles, and complex signal processing circuitry to determine phase relationships of scattered light signals.
Another well known velocity measuring technique is time-of-flight velocimetry, also referred to as transit time, two-spot, or two-focus velocimetry. According to this method, two beams of electromagnetic radiation with radially symmetrical intensity distributions are directed through a particle sampling volume. A particle, when passing through both beams, generates two pulse signals. A timing signal is initiated coincident with the first pulse and terminated coincident with the second pulse, for a "time of flight" measurement over a known distance, thus to yield particle velocity. This approach provides better signal-to-noise ratios than LDV for certain applications, and does not require light beam coherence. Further, since it involves measuring time rather than frequency, the typical time-of-flight application can employ lower cost signal processing circuitry. U.S. Pat. No. 4,251,733 (Hirleman, Jr.) describes a time-of-flight system that measures two velocity components and uses scattered radiation for estimating particle sizes. This system requires precise beam geometry, and analysis of the optical signals as to pulse width and pulse amplitude ratios.
An article by Mark P. Wernet entitled "Four-Spot Laser Anemometer and Optical Axis Techniques for Turbine Applications" (International Congress on Instrumentation in Aerospace Simulation Facilities--1987, pages 245-254) describes a four-spot time-of-flight laser anemometer system. This system requires two pairs of laser beams, with each pair having orthogonally polarized beams. The system employs four photodetectors, each configured to selectively receive light from one of the beams comprising the pairs. Signals from the two photodetectors corresponding to a given pair are subtracted to produce timing signals.
The time-of-flight technique is subject to error. For example, a particle can produce a signal sufficient for detection when passing through one of the beams, but not the other, either due to its trajectory or because of its size being at the borderline of detection. This occurrence, called a "single trigger event", can lead to an erroneous time-of-flight measurement when a signal from the arrival of a second particle is mistakenly identified as the departure signal of a first particle.
A second error for the two-spot technique occurs when one or more additional particles cross the beams between the initial and final signal for an initial particle. This creates ambiguity as to which pulses are associated with each particle for timing measurements, and is known as a "coincidence event". The frequency of this error is greatest-when only one photodetector collects light scattered from both beams. However, since coincidence events often occur when a faster travelling particle overtakes a slower particle during its flight between the beams, these errors can arise even when each beam is paired with its own light-photodetector.
Several patents to Batchelder et al, including U.S. Pat. Nos. 5,037,202; 5,061,070; and 5,133,602 disclose optical systems with partially overlapping beams directed through a fluid stream for measuring particle size, refractive index and position within the stream or flow. This system is based on an interferometric technique for sequentially measuring the change in the phase of each coherent light beam as particles are transported in the fluid. Two detectors simultaneously receive light emerging from the flow and provide their outputs to a summing amplifier for determining extinction, and to a difference amplifier for determining phase shift. Position measurement requires detailed analysis of the shapes and relative amplitudes of the phase signal.
It is well known to employ scattered or extinguished light to measure the "optical size" of individual particles. Optical size depends primarily on physical size, index of refraction and shape of the particle. Typically a gas containing the particles is drawn into a chamber to move the gas stream and particles through a region of substantially uniform light beam intensity. Light scattered by the particles is focused onto a photodetector, with the photodetector amplitude indicating optical size. Alternatively, in the light extinction approach, the photodetector output varies to indicate the reduction in beam power due to the particle.
Particle sizes also are determined based on their aerodynamic behavior. An aerodynamic particle size measuring device operates on the principle that a particle's velocity in an accelerating flow of air or other gas depends on its aerodynamic diameter. Aerodynamic diameter information is useful, in that aerosol behavior in the human respiratory system varies with aerodynamic particle diameter. Typically the device measures particle velocity in an accelerated flow, e.g. near a nozzle, near obstructions in the gas flow path, along curvature of the flow path or where two gas streams intersect. When a particle enters an accelerated flow, the "relaxation time" of its velocity--i.e. the time necessary for the particle velocity to coincide with velocity of the surrounding gas--depends on its mass, physical size and shape. When particle density is known, the measured aerodynamic size can be used to estimate particle mass. U.S. Pat. No. 3,854,321 (Dahneke) describes a particle beam device that includes a technique for sizing the particles.
Any of the above techniques for optical measurement of the velocity of bodies in a gas can be coupled with an acceleration of the gas flow in order to measure the aerodynamic sizes of the bodies. However, each of these known velocity measurement techniques has drawbacks associated with complexity and cost, or with measurement accuracy or errors due to coincidence and single trigger events.
Presently, the time of flight or two-spot method is preferred for measuring velocities in aerodynamic particle size measurement, since the signal processing is simpler and less costly. However, the velocity measuring errors described above are not only present in an aerodynamic size measuring device, but are amplified as to their effect, since particle mass varies with the cube of the aerodynamic diameter. Random errors can be mistakenly assumed to be large sized particles, and thus contribute to a disproportionately large error in measured mass. This error is sufficiently serious to prevent the use of aerodynamic particle size measuring in certain applications, such as determining cumulative erosol mass.
To address these errors, Heitbrink and Baron (An Approach To Evaluating and Correcting Aerodynamic Particle Size or Measurements for Phantom Particle Count Creation, American Industrial Hygiene Association Journal, July 1992, pp. 427-431) discuss a method of using two signal processors: a small particle processor and a large particle processor. These processors overlap in the aerodynamic particle diameter range of 5.2-15.4 micrometers.
The difference in particle counts in the overlap region is used to estimate an upper limit in the number of phantom particles from the small particle processor. The authors caution that correlation does not remove statistical noise caused by the detection of phantom particles, and further advise that the large particle processor responds to coincidence by underestimating the particulate concentration.
Therefore, it is an object of the invention to provide an optical particle characterizing system employing the time-of-flight technique in a manner to enhance reliability by avoiding the errors traditionally associated with this technique.
Another object is to provide, in time-of-flight velocity systems and in aerodynamic particle size measuring systems, signal processing circuitry to improve correction for particle coincidence and single trigger events, thus to improve measurements of particle concentration.
Another object is to provide a system for characterizing particles as to size, velocity and other traits over a wide dynamic range of electro-optic signals.
A further object is to provide an aerodynamic particle measuring system in which particle mass values are considerably more accurate, due to the reduction in velocity measurement errors.
Yet another object is to provide an optical system for determining a particle's optical size and aerodynamic size, and for applying these sizes in combination to determine further particle characteristics.